the terrasar-x ground segment

IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 48, NO. 2, FEBRUARY 2010 623

The TerraSAR-X Ground SegmentStefan Buckreuss and Birgit Schättler, Member, IEEE

Abstract—TerraSAR-X, the first national German remote-sensing satellite, was launched on June 15, 2007. It carries anX-band high-resolution synthetic aperture radar (SAR) instru-ment featuring imaging modes like StripMap, ScanSAR, and, par-ticularly, SpotLight in a variety of different polarization modes.Primary mission goal is the provision of both science and com-mercial users with a variety of products from advanced SARmodes. The TerraSAR-X Ground Segment, which is provided bythe German Aerospace Center (DLR), constitutes the central ele-ment for controlling and operating the TerraSAR-X satellite, forcalibrating its SAR instrument, and for archiving the SAR data,as well as generating and distributing the basic data products.This paper depicts the ground-segment layout and describes itsmajor elements. The ordering and product-generation workflow ispresented. It introduces the applied prelaunch integration, testing,verification, and validation approach, a major key to the comple-tion not only of the SAR technical-verification program but alsothe operational qualification of the ground segment itself withinthe commissioning phase.

Index Terms—Ground segment, remote sensing, synthetic aper-ture radar (SAR), TerraSAR-X.

ACRONYMS

DIMS Data and information management system.DRA Dual-receive antenna.EOWEB Earth observation data web access.GMTI Ground moving-target identification.HS High-resolution SL.IOCS Instrument operations and calibration segment.ISDN Integrated services digital network.ITVV Integration, technical verification, and validation.LCT Laser communication terminal.LEOP Launch and early orbit phase.MOS Mission operations segment.NSG Neustrelitz Ground Station, Germany.PGS Payload Ground Segment.SC Scan synthetic aperture radar (ScanSAR) mode.SL SpotLight mode.SM StripMap mode.TMSP TerraSAR-X Multimode SAR Processor.TR Transmit/receive.TOR Tracking occultation and ranging package.TSXX TerraSAR-X Exploitation Infrastructure.UPS Universal Polar Stereographic.UTM Universal Transverse Mercator.

Manuscript received February 27, 2009; revised July 14, 2009. First pub-lished November 3, 2009; current version published January 20, 2010.

S. Buckreuss is with the Microwaves and Radar Institute (HR), GermanAerospace Center (DLR), 82230 Wessling, Germany (e-mail: [email protected]).

B. Schättler is with the Remote Sensing Technology Institute (IMF), GermanAerospace Center (DLR), 82230 Wessling, Germany.

Digital Object Identifier 10.1109/TGRS.2009.2031432

I. INTRODUCTION

THE TERRASAR-X Ground Segment is the central facilityfor controlling and operating the TerraSAR-X satellite,

for calibrating the SAR instrument and maintaining its long-term system performance, and for receiving and archiving theSAR data as well as generating and distributing the user-dataproducts.

In the context of the entire TerraSAR-X system, shownin Fig. 1, the Ground Segment interfaces with the following:

1) the Space Segment, representing the satellite bus, and theSAR instrument provided by EADS Astrium GmbH aswell as the secondary payload:a) the LCT, a technology demonstrator for optical data

transfer in space designed by TESAT;b) the TOR experiment, prepared by the Geosciences

Research Center in Podsdam (GFZ) in collaborationwith the Center for Space Research of the Universityof Texas.

2) the Commercial Service Segment, respectively, theTSXX, also including Direct-Access Stations provided byInfoterra GmbH;

3) the Science Service Segment coordinated by the GermanAerospace Center (DLR);

4) the Science and Commercial User Segment.The Ground Segment is built up by and operated at the DLR.It is organized in three major parts:1) the MOS, provided by the German Space Operations

Center (GSOC);2) the IOCS, provided by the Microwaves and Radar

Institute;3) the PGS, provided by the Remote Sensing Technology

Institute in cooperation with the German Remote SensingData Center (DFD).

It partly consists not only of national infrastructure furtheroptimized for the mission needs but also includes a numberof dedicated subsystems to specifically serve the TerraSAR-Xmission [1].

II. TerraSAR-X IMAGING MODES

AND PRODUCT PORTFOLIO

From a SAR technical point of view, the generation anddistribution of the SAR user products is a central drivingrequirement for the ground segment.

A. Basic Products

The base for the standard SAR product generation are fouroperational imaging modes, namely:

1) StripMap configuration in the 3-m resolution class and ascene size of 30 km × 50 km (range × azimuth);

0196-2892/$26.00 © 2009 IEEE

624 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 48, NO. 2, FEBRUARY 2010

Fig. 1. TerraSAR-X System consisting of the space segment, ground segment,science- and commercial-service segment, and the user segments.

Fig. 2. TerraSAR-X basic imaging modes StripMap, normal and high-resolution SpotLight, and ScanSAR mode.

2) four-beam ScanSAR configuration with an 18-m resolu-tion and a scene size of 100 × 150 km (range × azimuth);

3) two sliding SpotLight configurations with an azimuthresolution of 1 to 1.7 m and a scene size of 10 × 5, . . . ,10 km (range × azimuth).

The full performance incidence-angle range covers 20◦ to45◦ for StripMap and ScanSAR and 20◦ to 55◦ for both SLs(see Fig. 2).

In addition to single polarization, data from StripMap andboth SLs are offered as dual-polarization variants but at a costof a double-sized azimuth resolution and a halved swath widthin SM.

The basic products available from the nominal imaging andpolarization configurations are specified in [6] and consist ofthe following:

1) single-look phase-preserving complex product in slant-range presentation and the derived detected multilookvariants;

2) multilook detected product in ground-range projection;3) geocoded ellipsoid-corrected product in Universal Trans-

verse Mercator (UTM) (Universal Polar Stereographic[UPS]) projection;

4) enhanced ellipsoid-corrected product in UTM (UPS)projection.

The multilook variants are offered in two flavors, the spa-tially enhanced and the radiometrically enhanced, whereby theScanSAR products are available as radiometrically enhancedvariant only. Geocoded ellipsoid-corrected projection is per-

formed using an average scene height; enhanced ellipsoid cor-rection and projection use a digital elevation model.

In addition to basic products from the standard high-resolution SL with its 150-MHz acquisition bandwidth, exper-imental products stemming from acquisitions with 300 MHzin single-polarization mode were successfully qualified andcharacterized in the commissioning phase and are operationallyavailable as well for the user community.

Detailed results from the product characterization and verifi-cation are also given in [6].

B. Experimental Modes and Products

Even if the basic products already support a wide range of ap-plications ranging from high-resolution small-scale imaging inthe SL to medium-scale applications typical for StripMap andScanSAR imagery, the azimuth-steering and beam-switchingcapabilities of TerraSAR-X combined with the extreme flexi-bility in instrument commanding are further exploited to opera-tionally support the generation and distribution of experimentalproducts and data from experimental modes comprising, e.g.,twin polarization, aperture switching, along-track interferome-try, and TOPSAR. The most prominent one is the experimentalquad-polarization product which is based on the DRA config-uration, i.e., the technical capability to split the antenna intotwo halves and to operate the redundant chain in parallel to theprimary.

III. TerraSAR-X GROUND-SEGMENT ELEMENTS

AND MAJOR FUNCTIONS

The requirements, as outlined in Section II, lead to the defin-ition of the three segments constituting the ground segment forTerraSAR-X, namely:

1) MOS—Mission Operations Segment;2) IOCS—Instrument Operations and Calibration Segment;3) PGS—Payload Ground Segment.

They are listed next in terms of their main elements. Theirtypical interaction during execution of a user order for futureproduct generation is described in Section IV.

A. MOS

The MOS operates from the GSOC in Oberpfaffenhofen,Germany. Its main functions are as follows:

1) Provision of S-band telemetry, tracking, and commandground-station networks suitable for the various missionphases: The station in Weilheim, Germany, is used forroutine activities. Additional stations are utilized duringLEOP and for contingencies;

2) Provision of a Central Checkout System to support thesatellite checkout during assembly, integration, verifica-tion, and testing: The Central Checkout System providesan interface to the satellite or to the spacecraft simulator.Development and test platform for various MOS elements(e.g., telemetry/telecommand display system);

BUCKREUSS AND SCHÄTTLER: TERRASAR-X GROUND SEGMENT 625

3) Satellite Monitoring and Control: Telemetry process-ing and monitoring, commanding, housekeeping, dataprocessing and archiving, data distribution;

4) Mission Planning and Scheduling: Daily planning ofradar data acquisition and downlink related activities,generation of command sequences [7];

5) Key-Management Facility: Generation and distribution ofencryption/decryption keys, according to the planned dataacquisitions and downlinks;

6) Flight Dynamics: Orbit and attitude determination, orbitprediction, maneuver planning. Provision of orbit andattitude products as input for SAR processing [8].

B. IOCS

1) Instrument Operations: The instrument operations com-prises the following tasks [9], [10]:

a) Radar Parameter Generation and Instrument Command-ing: Based on user input, the mission-planning systemgenerates requests for data takes. Each request is ana-lyzed, and the appropriate sequence of radar parametersexpressed as engineering values (pulse-repetition fre-quency, data window position, etc.) will be generated.This sequence is then transformed into macro commandsby the command generator and finally transferred into theinstrument binary language;

b) Instrument Monitoring: Housekeeping values which aredownlinked via S-band are checked and analyzed, bothautomatically and manually, to examine instrumenthealth;

c) Data-Take Verification: The correct execution of thecommanded data take is verified via the correlation ofthe monitored instrument operation with the commandedsequence and an examination whether statistics andprominent radar parameters of acquired data are withinmargins.

2) Calibration Segment: The calibration relates the SARimage intensity to radar backscattering coefficients and pro-vides information about the accuracy of this relationship. Es-timation and removal of all system-related influences result inpure object signatures. The goal is to model the relationshipbetween geophysical parameters and measured backscatteringcoefficients. Such quantitative analyses and the developmentand interpretation of models in different geophysical applica-tions require calibrated data [11].

The overall system calibration includes the following:

1) compensation/correction of known system errors in am-plitude and phase;

2) georeferenced transformation of SAR image data tobackscatter coefficients within an estimated error;

3) internal calibration;4) external calibration;5) accomplishment of calibration campaigns;6) development and procurement of calibration targets;7) calibration evaluation;8) delivery of calibration information to be used during SAR

product generation at the PGS.

C. PGS

1) DIMS: The DIMS is a multimission infrastructure of theDFD providing user-information services, order and productioncontrol, product library, and product delivery [17]. It is throughthe EOWEB component that a user is enabled to order a futureproduct for the acquisition of a new data take and its subsequentprocessing to a basic product. The user may also search thecatalogue for already existing data takes and thus request theirprocessing in the form of a catalogue order. A major DIMS taskis the organization of all requests derived from the user ordersin the so-called PGS request workflow.

2) NSG: Major TerraSAR-X tasks of the multimission NSGin Germany, operated by the DFD are the S- and X-band datareception, and, since recently, also the S-band uplink support.NSG operates several 7.3-m antenna systems, multimissionhigh-rate demodulators, and data-capture systems. All thesesubsystems are connected via matrices allowing a flexible sig-nal routing and device usage. Signal routing and controlling isperformed by the NSG monitoring and control system. All data-reception activities are planned in advance to a downlink event;the needed system parts are set up. Following a data-receptionprocess, comprehensive data quality and status information aregathered; a reception report is archived. This leads to a fullyautomated and multimission-harmonized reception chain [18].

The data-capture system used for X-band is the MDA-manufactured Direct Archive System which ingests the300-Mb/s serial data stream and finally reconstructs the indi-vidual data-take files embedded in a downlink in the instrumentsource packet format.

3) Transcription System: The Transcription System is aTerraSAR-X specific subsystem developed by DFD. It es-sentially performs data decryption and—in case of par-tially replayed data takes or data takes acquired in DRAconfiguration—the gathering of the various data files belongingto the same data take.

4) TMSP: The TMSP is the central PGS element [20] incharge of screening the SAR data received at NSG and ofgenerating the L0 products for long-term archiving as well asgenerating all basic products from the various imaging andpolarization modes [21]–[23]. It has been specifically devel-oped for the TerraSAR-X mission by the Remote SensingTechnology Institute using a generic “one fits all” approach,i.e., the core SAR correlator module provides phase-preservingsingle-look complex data sets in slant-range from all imag-ing modes. The derivation of the multilook detected productvariants is consistently based on this interim processing stage.A DFD-provided geocoding plug-in which performs ellipsoidcorrection and map projection is incorporated in the TMSP.

The TMSP is implemented in C++ and is scalable from smallworkstations to multi-CPU server systems. Parallel processingis achieved by applying multithread techniques.

The TMSP is also used as customer-furnished item by allDirect-Access Stations, which are a component of the com-mercial TSXX, thus ensuring that all TerraSAR-X users haveaccess to basic products identical not only in terms of productquality and format but also in terms of applied SAR processingtechniques.


In addition to basic and experimental product generation, theoperational TMSP is also able to process SAR data stemmingfrom a variety of experimental TerraSAR-X modes usually re-sulting in nonnominal imaging configurations [28], e.g., for spe-cific evaluations during the commissioning phase. It is throughthis feature—in combination with the high instrument com-mand flexibility operationally exploited by IOCS—that newproducts can be easily tested parallel to the nominal operationalinstrument commanding and SAR data-processing chains.

5) Processing System TMSP: The TMSP is integrated witha control entity which is responsible for scheduling and moni-toring all processing runs. It has to provide the TMSP with allneeded input data, i.e., SAR data as well as auxiliary products,like orbit and attitude, and selected housekeeping data.

The processing system TMSP is hosted by a mission-specific hardware and storage environment. SAR processingis performed on eight SUN V890 computers, each equippedwith eight double-core CPUs (1.5 GHz each). A storage-areanetwork enables fast data access from processing nodes. Thecontrol component is also in charge of scheduling and loadbalancing the processing hosts.

6) SAR Product Quality-Control System: An automaticproduct quality control is performed by the TMSP for eachof its generated products. If a product fails this online qualitycheck, it is sent into the offline interactive quality control; theotherwise automatic delivery to the user is set on hold andresumed only upon approval by the quality operator.

7) SAR Data Workflow: The PGS elements as described inSections III-C2–C6 contribute to the so-called SAR data work-flow [19] which runs asynchronously and follows a strict data-driven approach from the reception of the X-band downlinkup to the long-term archiving of the generated L0 products.Data-driven means that external data arriving at PGS triggerthe single-processing steps of the PGS elements. These externaldata specifically comprise auxiliary products (orbit, attitude,extracted house-keeping) and information about the downlinkcontent in addition to the SAR raw data itself. Each individualelement works autonomously and independent from the othersas long as new input data arrive and sufficient local data-storagecapability for its output data are available.

IV. GENERALIZED OVERALL ORDER WORKFLOW

The typical workflow based on the order of a science userstarts with the response to an announcement of opportunitystimulated by the TerraSAR-X science coordinator and endswith the delivery of an ordered so-called “future product.” Theterm “future” denotes that the requested acquisition does notyet exist in the archive but has to be acquired “in future.”Therefore, the desired acquisition parameters must be specifiedby the science user within an order process, finally activatingthe image acquisition and its subsequent processing followedby the product delivery.

The precondition to launch the order workflow is the assign-ment of the status “scientific use” via a selection process. Here,any utilization of TerraSAR-X data and products that is nottargeting a commercial profit-oriented use is a “scientific use.”Not only national and international education and research

Fig. 3. Overall workflow from science proposal to product delivery.

institutions but also companies involved in research and devel-opment projects are invited to submit proposals. All proposalspass an evaluation procedure, and accepted ones get the status“scientific use” and access to a specific quota of TerraSAR-Xdata and products [3]. More information is provided in theTerraSAR-X science plan [4].

The generalized overall workflow for the order of a scienceuser is described in the following and also briefly shownin Fig. 3.

1) The customer, respectively, the science user submits aproposal, which is the response to an announcement ofopportunity, to the science coordinator. The proposal canbe submitted electronically via the web presence of theScience Service Segment [5].

2) The science coordinator evaluates the proposal accord-ing to the rules quoted in the science plan. In case anonscientific use of the data is obvious, the customer isasked to contact Infoterra GmbH who provides supportfor commercial users.

3) In case the proposal is accepted and, thus, the status“science use” of the data is gained, the PGS is informedto enable the user account at the DIMS ordering control.

4) A notification of approval is sent to the customer whomay place his future orders from now on using the PGSEOWEB.

5) PGS handles the orders according to its order-management process and issues an acquisition request tothe mission planning system at MOS.

6) Based on a set of rules accounting for the acquisition pri-ority, timeliness, data-take location, conflicting maneu-vers, and other resources, such as onboard memory anddownlink capacity, the mission-planning system finallygenerates a timeline of instrument commands to be sentto the satellite.


7) Embedded in the mission-planning system is the so-called request-to-command-converter “R2CC,” providedby the IOCS, which ingests the mission-planning systemgenerated data-take requests specifying the data-take lo-cation, incidence angle, imaging and polarization mode,etc. In a first step, the radar-parameter generator of theR2CC transforms these parameters into radar parametersexpressed as engineering values, e.g., pulse-repetitionfrequency in hertz, range delay in seconds, receive gain indecibels, and so on. The radar parameters are convertedthen by the instrument-command generator module ofthe R2CC into binary instrument commands which canbe understood by the onboard computer controlling theradar instrument. In doing so, the adjustment of the radarinstrument is optimized individually for each data take inorder to achieve the optimum image quality.

8) The set of binary instrument commands is then returnedto the mission-planning system in order to refine thecalculation of the consumption of the onboard resources.

9) The mission-planning system generates the timeline withthe command sequences and informs PGS productioncontrol about the planned data takes.

10) The Monitoring and Control System performs the com-mand uplink via the Weilheim ground station.

11) The SAR instrument carries out the commanded datatakes at the desired location with the individually calcu-lated instrument-parameter settings and stores them in theonboard solid-state mass memory.

12) During NSG contacts, the X-band replays are prepared bythe onboard Telemetry Formatter and Encryptor unit andthe downlink is performed.

13) The X-band downlinks are received at the NSG allocatedto PGS. The direct archive system performs the framesynchronization, descrambling, and Reed–Solomon de-coding and reconstructs the individual data takes from thereplays in instrument source packet format.

14) The PGS Transcription System performs the data de-cryption and assembles—in case of partially replayeddata takes and/or data takes acquired in the DRAconfiguration—the various data files which belong to thesame data take.

15) The PGS Processing System TMSP then systematicallyperforms the screening and L0 product generation foreach incoming new data take. This includes raw-dataquality and SAR parameter determination, calibration,and noise-pulse evaluation, Doppler centroid determina-tion, and a systematic single-look complex-image gener-ation to quick-look generation.

16) Based on the mission-planning-system-provided data-take planning information and well in advance to a down-link event, production requests for the Processing SystemTMSP are released by the PGS Production Control.

17) The basic product generation is initiated for executionas soon as the needed orbit and attitude products areprovided by MOS Flight Dynamics.

18) The TMSP-generated basic products are provided for thescience user by the PGS User Services at the productdelivery pickup point.

19) The TMSP-generated L0 products are archived in thePGS Product Library and the EOWEB catalogue is filledwith the L0 metadata to enable a later basic-productordering from the catalogue.

V. GROUND-SEGMENT INTEGRATION

AND TESTING APPROACH

A. Rationale

The ground-segment integration and testing rationale is atailored version of the standards laid down by the European Co-operation for Space Standardization [2] and applies to phases Cand D of the project. The entire process is denoted as integrationand technical verification and validation, furthermore abbrevi-ated as ITVV.

The ITVV rationale basically pursues a bottom-up approach.Integration and testing starts at component level and dilatesto subsystem, subsegment, and then to ground-segment level.Finally, it comprises the various segments contributing to theoverall system as shown in Fig. 1.

Testing of the subsegments is performed through individualITVV processes according to dedicated plans. Each subsegmenthas to undergo a number of acceptance tests to demonstrate theconformance of its elements (subsystem or component) withthe underlying requirement and design specifications as well asthe specifications for its interfaces with other elements. Passingwell-defined acceptance tests qualifies the subsegment elementsfor integration and testing on ground-segment level. The groundsegment ITVV plan organizes the integration and testing onground-segment level as further described in Sections V-Bto E.

The performance of a prelaunch end-to-end testing startingwith user-order input and ending with the delivery of thegenerated products, including the space segment in additionto the various ground segment elements, was set as a primaryobjective for the ground-segment validation. Its execution isdescribed in Section VI.

B. Test Assemblies and Test Data Base

Verification and validation testing is based on testable enti-ties, which are so-called assemblies. Each assembly involvesa well-defined set of elements (subsystems or components) inthe MOS, IOCS, and PGS subsegments. These elements arebroken down into configuration items which are defined in theTerraSAR-X ground-segment project database in the configura-tion database section. The test assembly definition and execu-tion process is completely handled via the project database.

Each test assembly is described in the same way: Overviewdescriptions detail purpose and scope, pretest activities to becarried out, test tools and test data to be used, and list possibledependencies on other test assemblies. Most important is the listof configurable items as input for configuration management.A given test assembly contains a number of test cases alsospecified according to a unified definition scheme. Each testcase is prefaced with an overall description, input data, test-startconditions; pass/fail criteria are listed, and the person in charge


for the test case is named. Individual test steps are definedtogether with their expected outcome.

When executing a given test assembly, a so-called test-assembly version has to be generated. Each involved configu-ration item is then marked with its current-version informationfrom the configuration database, thus, linking test execution andground-segment configuration management. Setting up a newtest-assembly version in the database leads to the automaticgeneration of a test-reporting template out of the underlyingtest-case definition. This template is to be filled out duringexecution of a specific test case. For each test step, the obtainedresult is documented and cross checked against the expectedone. The closing out of a test assembly depends on the success-ful completion of all test cases indicated by the verdict “pass.”

The benefits of the database-supported test-assembly defini-tion and execution are manifold: The complete testing processis linked with the ground-segment configuration management.The engineers are guided through the complete test definitionand execution procedure. This helps ensure that all requiredinformation is gathered. The editorial effort is minimized,specifically for tests to be repeated. The detailed and up-to-date execution status of all test assemblies and test cases canbe assessed anytime not only by the test engineers but also bythe project and quality management staff. Overview summariesmay also be generated at any time. The automatic generation oftest documentation, e.g., for reviews, is supported.

C. Integration

The subsegment elements are integrated into their opera-tional environment or into an environment equivalent in termsof the interfaces, inputs, and outputs. They are put under versioncontrol by assigning the version information to the underlyingconfiguration items.

D. Technical Verification

Focus of the technical verification on ground-segment levelare the interfaces between the MOS, IOCS, and PGS sub-segments as described in the mutual interface control doc-uments. A verification assembly thus, usually, comprises arather small number of interface partners interacting acrosssubsegment boundaries with respect to a given interface class.Typical example: An orbit and attitude product-handling as-sembly comprises the generation and provision of orbit andattitude products by MOS flight dynamics and their subsequentingestion and processing by the PGS Processing System TMSP.In total, about 20 verification assemblies are performed on aground-segment level.

The same approach was taken for intersegment verificationtesting with the space segment and the commercial-servicesegment based on the ground segment versus external interfacecontrol documents specifying the interfaces to these externalentities.

An important preparation activity for the verification phasewas the mutual exchange of interface-item samples betweenthe interface partners even before the ITVV process on ground-segment level had been started. This helped to detect and to

clean up interface definition inconsistencies or implementationincompatibilities well in advance of formal testing.

E. Validation

The objective of the validation phase is the confirmation thatthe ground segment is ready for use. Thus, the focus of thisphase is to demonstrate fully functional end-to-end workflowsand operational scenarios. Consequently, a small number ofvalidation assemblies (about five) are executed comprising notonly the entities interacting across subsegment boundaries butalso those entities contributing subsegment internally to theoverall workflow under test. Specifically, significant use of thealready integrated satellite was made.

VI. PRELAUNCH VALIDATION TESTING

OF MAJOR WORKFLOWS

As already quoted in Section V, a major goal of the validationprocess was to demonstrate the fully functional end-to-endworkflow, starting with user orders and ending with the deliveryof the generated SAR products under most realistic operationalconditions.

A. Overall SAR Data Ordering, Commanding, Acquisition,Reception, L0 Product Generation, and Archiving

1) Test Scenario: This test assembly was dedicated to vali-date the workflow through the ground segment for the ordering,planning, commanding, acquisition, reception, and processingof SAR data to L0 products and, thus, comprised the essentialsteps 5 through 15 from the generalized workflow described inSection IV.

A most important aspect of this validation test was the usageof the satellite itself with the integrated and fully functionalinstrument. It was commanded from the mission control roomat GSOC, yet a connection via integrated services digital net-work was utilized instead of the S-band telemetry/telecommandlink. Executing the commanded data takes was an indispensableprerequisite to obtain SAR test data consistent with the data-take requests. Thus, most realistic on-ground test conditions forthe workflow steps 10, 11, and 12 were reached.

The requested SAR data takes were for a typical mission day,e.g., as expected during the PGS checkout in the commissioningphase. Not only all nominal imaging and polarization modes inright-looking configuration were exercised but also the exper-imental modes like the DRA configuration and the 300-MHzbandwidth. Near-real-time data takes and partial replays wereincluded. All data-take requests were stimulated by nomi-nal user orders submitted via EOWEB. Nominal NeustrelitzX-band contacts were used to set the downlink margins. In total,over 50 data takes were acquired and replayed in six downlinksessions.

The data were recorded from the satellite using the so-called Data Evaluation Unit which is part of the space segmentcheckout equipment. They were stored in raw binary format ona portable disk for transport to the NSG and ingestion into theNeustrelitz Downlink Simulator for generation of the 300-Mb/sserial-data stream. Via a test modulator and a fiber-optic link,


Fig. 4. Workflow for testing the SAR data ordering, commanding, acquisition,reception, and L0 product archiving. The acting subsystems are marked inorange; the test steps are marked with yellow number tags according to thefollowing sequence: 1) Ordering at EOWEB and feasibility check of requesteddata take. 2) Generation of acquisition request. 3) Acquisition-request pro-vision. 4) Provision of capability and availability information. 5) Timelineand save stack-file generation. 6) Offline check of data-take command sets.7) Simulated command uplink (transmission via ISDN). 8) Generation anddistribution of key information file. 9) Downlink information file provision.10) Key information file provision. 11) Data-take execution (on-ground, insideintegration hall). 12) Simulated raw data reception (data transport on portabledisk). 13) Transcription. 14) L0 product generation. 15) Archiving of L0product.

the data were finally routed into the NSG X-band antennatriggering the operational PGS SAR data workflow.

The IOCS auxiliary product and the MOS orbit-data mes-sage and orbit and attitude products (simulated based on thereference orbit with a repeat cycle of 11 days) as needed for thePGS SAR processing were provided offline.

Offline verification checks were performed to check on thefollowing.

1) The acquired data take are consistent to the user order interms of mode, beam, geographical location.

2) The mission timeline is compliant with onboard memory,on-ground downlink capacities, etc.

3) The product-location extent and accuracy in SpotLightand high-resolution SL is as required.

Fig. 5. Image shows a quad-polarization acquisition over Munich, acquired at2009-05-02T05:34:54. The copolar channels are coded in red (HH) and green(VV), the cross-polar channels in red (HV + VH). All channels are relativelyscaled to a common mean value. The different vegetated and urban areas areclearly distinguished by their polarimetric signatures.

2) Affected Ground and Space-Segment Subsystems: Thefollowing ground subsystems were affected by this validationassembly:

1) MOS:a) Mission-Planning System;b) Monitoring and Control System;c) Key-Management Facility;d) Flight Dynamics.

2) PGS:a) DIMS:

i) DIMS User-Information Services (EOWEB andproduct delivery);

ii) DIMS Ordering and Production Control;iii) DIMS Product Library.

b) Neustrelitz Receiving Station;c) Transcription System;d) Processing System TMSP;e) SAR Product Quality-Control System.

3) IOCS:a) Instrument Command Generator Prototype (offline);b) Long-Term Data Base;c) Auxiliary Data Formatter.

4) Space Segment:a) Radar Instrument;b) Onboard Computer;c) Solid-State Mass Memory.


Fig. 6. Leftmost image shows part of a high-resolution SpotLight image from New York with Manhattan in its center. It was acquired in single polarization(HH) with 300-MHz bandwidth on July 1, 2009 and also on June 6, 2009, at 22:42:55, at an incidence angle of 29◦. The Shearson Lehman Plaza building isrecognized as a high-rise building outstanding among smaller ones at the right of the image (Greenwich Street). The two images in the center show its amplitudeand interferometric phase. By counting about 3.6 visible fringes and applying the height conversion factor of 4.11 m for this interferogram, its height is measuredto be 148 m which is very close to its reported height of 151.2 m. The rightmost image shows the Brooklyn Bridge and the Manhattan Bridge, with their clearlyvisible bridge structures in an amplitude, coherence, and phase overlay. The Brooklyn Bridge elaborates a remarkable fringe pattern, whereas coherence is totallylost in the middle of the Manhattan Bridge. The interferogram was generated with DLR’s GENESIS interferometry system.

3) Test Tools: The Central Checkout System and the DataEvaluation Unit of the space segment were needed to supportcommanding of the instrument and payload data recording.

At PGS side, the downlink simulator at NSG performed theconversion of the recorded data into a 300 MB/s serial-datastream played back with the required clock speed and stabilityfor direct ingestion into a test modulator and, from there, via afiber-optic link into the receiving antenna.

4) Test Cases: The SAR data workflow as exercised withinthis assembly was broken down into test cases covering the es-sential (mostly time-coherent) processing steps executed at theinvolved subsegments PGS, MOS, and IOCS as shown in Fig. 4.

5) Validation-Test Outcome: An end-to-end execution ofthe complete validation-test assembly including the satellitewas done twice; the first time in July 2006 and the secondtime in December 2006. The second execution was the close-out for a number of nonconformances as detected during thefirst run. These were mainly related to the generation andexecution monitoring of the generated mission timeline andimprovements with respect to the product-location accuracyand extent. The repetition of the full test assembly also providedan excellent opportunity to further exercise the end-to-end SARdata workflow under most realistic operational conditions.

B. Basic-Product Ordering, Generation, and Delivery

This test assembly was a straightforward extension of the pre-vious one by including science and commercial basic-productordering as well as their generation and, finally, delivery. It wassuccessfully performed in connection with the rerun of the SARdata-workflow validation assembly from Section VI-A.

C. Commercial Direct Access Ordering and Workflow

A second extension of the SAR data-workflow validationassembly was the inclusion of commercial direct-access orders,

through which data takes are requested for downlink at aTSXX direct-access station. Such orders were also includedinto the rerun of the first assembly leading to the execution andrecording of these data takes as well for further processing bythe Direct Access Reference Station located at Neustrelitz.

Since the Direct Access Reference Station uses an NSGantenna as front end for X-band data reception, the raw binarydata were ingested in the same way as the NSG downlinksand further processed by the standard TSXX Direct AccessTerminal. Moreover, on TSXX side, the full workflow includingthe TSXX internal handling of production requests and thebasic-product generation was exercised.

On the ground-segment side, the provision of a direct-accessstation with the appropriate downlink and decryption informa-tion was proven.

D. Validation-Testing Achievements

According to our knowledge, TerraSAR-X is the first SARmission to incorporate such a comprehensive end-to-end pre-launch validation-test program as outlined in Sections V-A–Cfor its ground segment. Even though its execution requiredconsiderable effort and on-ground testing time with the alreadyintegrated TerraSAR-X satellite, the gained advantages aremanifold; the saved in-orbit checkout time outnumbers the daysinvested on ground. From the first mission day on, all data-take requests and executions as well as all SAR productionruns were performed with the operationally installed ground-segment elements.

The first batch of user orders for the calibration and verifi-cation team was ingested via EOWEB a few hours after launchonly. Already, the first image on mission day 5 was included inthis order batch and, thus, subject to nominal mission planningand instrument commanding and nominal SAR processing bythe operationally installed TMSP. Neither a manual interactionnor a parameter tuning was required to obtain the first quick


look of the focused image demonstrating the functioning of thefull SAR chain in both the space and ground segment in a veryremarkable way.

The validation-test program certainly set the base to success-fully finish the operational qualification of the ground segmentwithin the given commissioning-phase time frame [12]. Fromearly in the commissioning phase on, the TSXX commercial-service segment was allowed to release future product orderswhich were fulfilled on a best effort basis. Moreover, access tocatalogue ordering was granted.

VII. CONCLUSION

A. Outlook

Despite the satisfactory results obtained so far, the groundsegment may never be considered to have reached a finalstate. The ground segment is always subject to an evolutionaryprocess, mainly driven by the extraordinary flexibility of theradar instrument. The latter offers the possibility to introducenew modes and to extend the portfolio by new experimentalproducts, such as the following:

1) the TOPSAR mode, already demonstrated during thecommissioning phase [24]–[26];

2) aperture-switching mode, enabling the along track inter-ferometry for traffic measurements [27];

3) DRA configuration for GMTI, currently under testwith promising results [28], [29] and quad-polarizationimaging.

Finally, the TanDEM-X mission [33] poses a great challengenot only from the scientific point of view but also for thepractical implementation of the required extensions into theoperational TerraSAR-X ground segment. For the minimizationof risks concerning the significance of test procedures and toensure efficiency in the project work, the established methodsare used as a standard henceforth.

B. Resume

It was a very remarkable achievement of the TerraSAR-Xmission, that only four days after launch on June 19, 2007,the first SAR image could be presented. Hereby, we couldclearly demonstrate the worthiness of the testing rationale andthe end-to-end testing program. In particular, the chance to gainaccess to the spacecraft for pretesting and including it in testscenarios together with the ground segment set the base for theaccomplishment of the in-orbit checkout and commissioningphase. Moreover, the production of the first image was nota single event; during the first ten days of the mission, allradar modes were checked out, and the preconditions for thefollowing commissioning phase were established.

The commissioning phase was passed, and the basic productswere released according to the Basic Product SpecificationDocument [6] on the end of December 2007.

On January 7, 2008, the operational phase was kicked offand the image production for scientific and commercial usershas been running extremely satisfactorily since then.

Due to the high resolution, the high radiometric stabil-ity and positioning accuracy the TerraSAR-X products are

TABLE IACTUAL TERRASAR-X INSTRUMENT AND CALIBRATION

PERFORMANCE MEASUREMENTS

well appreciated by the scientific and commercial community(see Figs. 5 and 6).

The stability of the radar instrument, the outstanding perfor-mance of the radiometric calibration, and the unique geometricaccuracy of the images are considerably outperforming thespecifications. A brief overview of actual performance numbersis given in Table I. An elaborate presentation of the calibrationresults is published in [11]–[14]. The overall system perfor-mance is addressed in [15].

The outstanding quality of the TMSP-generated products hasbeen shown in the frame of commissioning-phase verificationactivities and confirmed in the meantime by external users.Specifically, an absolute location accuracy below 1 m wasconfirmed in the end-user products. The product performanceis detailed in the Basic Product Specification [6].

These performance assets allow scientific and commercialapplications in many fields, such as repeat-pass interferometry[30], [31] persistent scatterer evaluation [32], and multitempo-ral analysis, with a quality unknown so far.

ACKNOWLEDGMENT

The authors would like to thank R. Werninghaus, TerraSAR-XProject Manager and W. Pitz, EADS Astrium space segmentProject Manager, who made the organizational commitment toassemble and specifically test the complete TerraSAR-X systemconsisting of both, space and ground segment, J. Mittermayer,D. Schulze, R. Metzig, U. Steinbrecher and M. Schwerdtof IOCS, W. Balzer, T. Fritz, M. Wolfmüller, E. Schwarz,E. Diedrich of PGS, H. Hofmann, E. Maurer and A. Codazziof MOS, who ensured the successful integration and testingof the ground segment by contributing many valuable ideasfor establishing the ITVV procedures, defined the detailed testprocedures, and rendered an outstanding service for planningand preparation of the commissioning phase, A. Schwab andC. Giese from the TerraSAR-X space segment team at EADSAstrium for their never-ending enthusiasm in providing ap-propriate instrument test data sets and the outstanding goodcooperation during prelaunch validation testing involving the


integrated TerraSAR-X satellite, and all the members of thespace and ground segment teams for their true willingness andcommitment to the TerraSAR-X project.

REFERENCES

[1] R. Werninghaus and S. Buckreuss, “The TerraSAR-X mission and systemdesign,” IEEE Trans. Geosci. Remote Sens., vol. 48, no. 2, pp. 606–614,Feb. 2010.

[2] “Space engineering, Ground systems and operations—Part 1: Principlesand requirements,” ECSS-E-70 Part 1A, Noordwijk, The Netherlands:ECSS Secretariat, ESA-ESTEC Requirements & Standards Division,2000.

[3] Data access and products. [Online]. Available: http://www.dlr.de/en/desktopdefault.aspx/tabid-4219/8885_read-16052/

[4] TerraSAR-X Science Plan, TX-PGS-PL-4001. [Online]. Available:http://sss.terrasar-x.dlr.de/pdfs/TSX-Science-Plan.pdf

[5] TerraSAR-X Science Service Segment Web Presence. [Online].Available: http://sss.terrasar-x.dlr.de/

[6] TerraSAR-X Basic Product Specification, TX-GS-DD-3302. [Online].Available: http://sss.terrasar-x.dlr.de/pdfs/TX-GS-DD-3302.pdf

[7] E. Maurer, F. Mrowka, A. Braun, M. P. Geyer, C. Lenzen, Y. Wasser,and M. Wickler, “TerraSAR-X mission planning system: Automated com-mand generation for spacecraft operations,” IEEE Trans. Geosci. RemoteSens., vol. 48, no. 2, pp. 642–648, Feb. 2010.

[8] R. Kahle, B. Kazeminejad, M. Kirschner, Y. Yoon, R. Kiehling, andS. D’Amico, “First in-orbit experience of TerraSAR-X flight dynamicsoperations,” in Proc. 20th ISSFD, Annapolis, MD, 2007.

[9] J. Mittermayer, U. Steinbrecher, A. Meta, N. Tous-Ramon, S. Wollstadt,M. Younis, J. Marquez, D. Schulze, and C. Ortega, “TerraSAR-Xsystem performance and command generation,” in Proc. EUSAR,Friedrichshafen, Germany, 2008.

[10] U. Steinbrecher, D. Schulze, J. Böer, and J. Mittermayer, “TerraSAR-Xinstrument operations rooted in the system engineering and calibrationproject,” IEEE Trans. Geosci. Remote Sens., vol. 48, no. 2, pp. 633–641,Feb. 2010.

[11] M. Schwerdt, B. Bräutigam, M. Bachmann, and B. Döring, “TerraSAR-Xcalibration results,” in Proc. EUSAR, Friedrichshafen, Germany, 2008.

[12] M. Schwerdt, B. Bräutigam, M. Bachmann, B. Döring, D. Schrank, andJ. Hueso Gonzalez, “Final TerraSAR-X calibration results based on novelefficient methods,” IEEE Trans. Geosci. Remote Sens., vol. 48, no. 2,pp. 677–689, Feb. 2010.

[13] M. Bachmann, M. Schwerdt, and B. Bräutigam, “TerraSAR-X antennacalibration and monitoring based on a precise antenna model,” IEEETrans. Geosci. Remote Sens., vol. 48, no. 2, pp. 690–701, Feb. 2010.

[14] B. Bräutigam, J. H. Gonzáles, M. Schwerdt, and M. Bachmann,“TerraSAR-X instrument calibration results and extension forTanDEM-X,” IEEE Trans. Geosci. Remote Sens., vol. 48, no. 2,pp. 702–715, Feb. 2010.

[15] J. Mittermayer, M. Younis, R. Metzig, S. Wollstadt, J. Márquez Martínez,and A. Meta, “TerraSAR-X system performance characterization and ver-ification,” IEEE Trans. Geosci. Remote Sens., vol. 48, no. 2, pp. 660–676,Feb. 2010.

[16] J. Mittermayer, B. Schättler, and M. Younis, “TerraSAR-X commissioningphase execution summary,” IEEE Trans. Geosci. Remote Sens., vol. 48,no. 2, pp. 649–659, Feb. 2010.

[17] M. Wolfmüller, D. Dietrich, E. Sireteanu, S. Kiemle, E. Mikusch, andM. Böttcher, “Data flow and workflow organization—The data manage-ment for the TerraSAR-X payload ground segment,” IEEE Trans. Geosci.Remote Sens., vol. 47, no. 1, pp. 44–50, Jan. 2009.

[18] H. Damerow, J. Richter, J. Schwarz, H.-J. Pannowitsch, and H. Maass,“Ausbau der multimission bodenstation neustrelitz für TerraSAR-X,” inProc. Deutscher Luft- und Raumfahrtkongress, Braunschweig, Germany,Nov. 6–9, 2006.

[19] B. Schättler, M. Wolfmüller, R. Reissig, H. Damerow, H. Breit, andE. Diedrich, “A description of the data-driven SAR data workflow inthe TerraSAR-X payload ground segment,” in Proc. IEEE IGARSS,Anchorage, AK, 2004, pp. 4543–4547.

[20] B. Schättler, H. Breit, T. Fritz, M. Eineder, E. Schwarz, H. Damerow,U. Balss, E. Boerner, and W. Balzer, “The TerraSAR-X payload groundsegment: Pre-launch status and performance,” in Proc. CEOS SARCalibration Workshop, Edinburgh, U.K., 2006.

[21] H. Breit, T. Fritz, U. Balss, M. Lachaise, A. Niedermeier, andM. Vonavka, “TerraSAR-X SAR processing and products,” IEEE Trans.Geosci. Remote Sens., vol. 48, no. 2, pp. 727–740, Feb. 2010.

[22] M. Stangl, R. Werninghaus, B. Schweizer, C. Fischer, M. Brandfass,J. Mittermayer, and H. Breit, “TerraSAR-X technologies and first results,”

Proc. Inst. Elect. Eng.—Radar, Sonar, Navig., vol. 153, no. 2, pp. 86–95,Apr. 2006.

[23] H. Breit, B. Schättler, T. Fritz, U. Balss, H. Damerow, and E. Schwarz,“TerraSAR-X payload data processing: Results from commissioning andearly operational phase,” in Proc. IEEE IGARSS, Boston, MA, 2008,pp. II-209–II-212.

[24] A. Meta, J. Mittermayer, U. Steinbrecher, and P. Prats, “Investigationson the TOPSAR acquisition mode with TerraSAR-X,” in Proc. IEEEIGARSS, Barcelona, Spain, 2007, pp. 152–155.

[25] A. Meta, J. Mittermayer, P. Prats, R. Scheiber, and U. Steinbrecher,“TOPS imaging with TerraSAR-X: Mode design and performance analy-sis,” IEEE Trans. Geosci. Remote Sens., vol. 48, no. 2, pp. 759–769,Feb. 2010.

[26] P. Prats, R. Scheiber, J. Mittermayer, A. Meta, and A. Moreira, “Process-ing of sliding spotlight and TOPS SAR data using baseband azimuthscaling,” IEEE Trans. Geosci. Remote Sens., vol. 48, no. 2, pp. 770–780,Feb. 2010.

[27] S. Suchandt, H. Runge, H. Breit, A. Kotenkov, D. Weihing, and S. Hinz,“Traffic measurement with TerraSAR-X: Processing system overviewand first results,” in Proc. EUSAR, Friedrichshafen, Germany, 2008.

[28] S. Suchandt, H. Runge, H. Breit, U. Steinbrecher, A. Kotenkov, andU. Balss, “Automatic extraction of traffic flows using TerraSAR-X along-track interferometry,” IEEE Trans. Geosci. Remote Sens., vol. 48, no. 2,pp. 807–819, Feb. 2010.

[29] M. Gabele, B. Bräutigam, D. Schulze, U. Steinbrecher, N. Tous-Ramon,and M. Younis, “Fore and aft channel reconstruction in the TerraSAR-Xdual receive antenna mode,” IEEE Trans. Geosci. Remote Sens., vol. 48,no. 2, pp. 795–806, Feb. 2010.

[30] N. Adam, M. Eineder, B. Schättler, and N. Yague-Martinez, “FirstTerraSAR-X interferometry evaluation,” in Proc. Fringe Workshop,Frascati, Italy, 2007, [CD-ROM] (ref. SP-649).

[31] M. Eineder, N. Adam, R. Bamler, N. Yague-Martinez, and H. Breit,“TerraSAR-X spotlight SAR interferometry,” IEEE Trans. Geosci.Remote Sens., vol. 47, no. 5, pp. 1524–1535, May 2009.

[32] N. Adam, M. Eineder, N. Yague-Martinez, and R. Bamler, “TerraSAR-Xhigh resolution SAR interferometry,” in Proc. EUSAR, Friedrichshafen,Germany, 2008.

[33] M. Zink, G. Krieger, H. Fiedler, I. Hajnsek, and A. Moreira,“The TanDEM-X mission concept,” in Proc. EUSAR, Friedrichshafen,Germany, 2008.

Stefan Buckreuss received the Dipl.-Ing. degree inelectronics from the Technical University of Munich,Munich, Germany, in 1988 and the Dr.-Ing. degreefrom the University of Stuttgart, Stuttgart, Germany,in 1994.

He has been with the Deutsches Zentrum für Luft-und Raumfahrt (DLR), Microwaves and Radar Insti-tute, Oberpfaffenhofen, Germany, since 1988 wherehe gained broad experience in SAR signal processingand motion compensation, antijamming, and inter-ferometry for DLR’s airborne SAR system E-SAR.

Within the scope of the international SRTM/X-SAR mission he developedthe online radar data analysis and quick-look processing tools for the X-SARsensor. Since 2002, he has been engaged in the German TerraSAR-X missionwhere he was responsible for the development of the instrument operationsand calibration segment until he was assigned the role of the ground-segmentIntegration Manager. Currently, he is the TerraSAR-X Mission Manager anddesignated Mission Manager for TanDEM-X.

Birgit Schättler (M’09) received the Diplomadegree in mathematics from the Bayerische Julius-Maximilians-Universität, Würzburg, Germany,in 1986.

From 1987 to 2000, she was with the DFD ofthe German Aerospace Center (DLR), Wessling,Germany. Since 2000 she has been with the Re-mote Sensing Technology Institute, DLR, where sheworked in the TerraSAR-X project as SAR SystemEngineer of the Payload Ground Segment (PGS)and was the PGS Technical Lead during the Com-

missioning Phase. Since 2008, she has been responsible for the integrationand acceptance testing of the TerraSAR-X ground-segment extension for theTanDEM-X mission.

the terrasar-x ground segment

Documents